1. Field of the Invention
A precision air gauge for measuring the geometry of fluid passages in a fuel injector.
2. Background Art
In the manufacture of fuel injectors, particularly fuel injectors for diesel engines, precision machining of plunger bores, control valve openings, fluid passages and other physical features of an injector pump body or an injector valve body are required. An example of a known injector with precision machined pressure distribution passages may be seen, for example, by referring to U.S. patent application Ser. No. 09/245,106, filed Jan. 29, 1999, which is owned by the assignee of the present invention. That injector, which is commonly referred to as a unit pump, includes an injector pump plunger mounted for reciprocation in a cylindrical bore in a pump body. The bore and the plunger define a pumping chamber, which is pressurized during a fuel injection event as the plunger is stroked by a cam follower driven by an engine camshaft. A control valve body, formed integrally with the pump body, includes a control valve opening that receives a control valve. The control valve opening is in communication with the pumping chamber and with a high pressure distribution circuit that communicates with an injection nozzle formed in a nozzle body.
In the manufacture of a unit pump of this kind, high pressure precision machined passages are required for connecting the pumping chamber with the control valve chamber and for connecting high pressure regions of the assembly to the injection nozzle. Dimensional control of the passages during precision machining of an injector of this kind is critical.
We are aware of prior art air gauges for measuring the quality and dimensions of a machined opening or a fluid passage wherein air pressure is introduced to the opening or the passage through an orifice in the air gauge. The characteristics of the machined opening or the passage can be detected by measuring the air pressure developed at the gauge orifice as a gauge probe is inserted into the passage or into the machined opening. The magnitude of that pressure can be used as an input signal for a pressure sensor to determine variations in the dimensions of the opening or the passage. For example, U.S. Pat. No. 4,704,896 discloses a probe that can be inserted into a drilled, blind opening or passage to detect whether the opening or the passage has internal threads.
Another example of a known air gauge using a probe to measure the characteristics of a machined opening is disclosed in U.S. Pat. No. 3,667,284. That measuring gauge includes a tapered bore with multiple radial jets that communicate with a central air passage. By measuring the back pressure developed at each jet, an operator can determine whether the opening is properly tapered. An equal back pressure at each jet position will indicate that the bore is properly tapered. If the bore is not properly tapered, the back pressure readings will vary.
Air gauges of the kind disclosed in prior art teachings are not practical for obtaining precision readings of the physical characteristics of a fluid pressure passage at precise gauge points. Attempts to use such air gauges to measure the characteristics of a fluid passage at precise depths using a trial-and-error technique generally are unacceptable and not practical for use in a high volume injector manufacturing environment. If an attempt is made to precisely control the depth of the probe using externally mounted gauge blocks, for example, the measurement routine becomes too complex to use on a shop floor in a high-volume manufacturing operation. Further, the results would not be precise enough to meet desired quality standards.
In the manufacture of an injector of the kind disclosed in the previously identified pending patent application, a long precision-machined passage must be drilled in an injector pump or control valve body to provide fluid communication between the control valve chamber and the source of high injection pressure at the pumping chamber. Following the precision drilling operation, the open end of the passage must be plugged to seal the passage against leakage during operation. For this purpose, it is preferred to use a pin, which is inserted into the passage following the machining operation. The pin can be formed with a shape memory alloy (SMA) and inserted in the opening with minimal pressure (for example, finger pressure). The pin then can be heated so that it will expand to provide a permanent seal. To be effective, the dimensions of the opening must be precise. For this reason, close dimensional tolerances at specified gauge points are required by quality control standards.
The air gauge of the invention includes a probe that can be inserted into a machined fluid pressure passage in the pump or control valve body. The probe extends from a probe body that receives a sleeve secured to the body at a fixed position with respect to the probe. A depth control bushing, according to one embodiment of the invention, is secured to one end of the sleeve by a lost-motion connection that will permit relative movement between the bushing and the sleeve.
A spring is located between the bushing and the probe body so that the probe body normally is biased against a first stop established by the lost motion connection. When the probe is inserted in the passage, a first gauge point is established when the bushing engages a stop surface on the pump or control valve body. The sleeve then can be moved to advance the probe within the passage until a second stop on the sleeve engages a stop surface on the bushing. In this way, two precise gauge points are established in the opening, and air pressure measurements are taken at each point. By comparing the measurements, it can be determined whether a desired degree of taper in the passage is present following the machining operation. Further, out-of-roundness of the passage and deviations in diameter for the passage can also be detected. These characteristics of the pressure passage, particularly measurements of the taper of the passage, can readily be obtained with the required precision and with repeatable inspection results.
In an alternate embodiment of the gauge of the invention, a stop surface on the bushing engages a stop surface on the probe body when the probe is advanced from the first gauge point to the second gauge point.
According to still another alternate embodiment of the invention, multiple positions of the probe relative to the bushing are established by a detent mechanism rather than by engageable stop surfaces on the bushing and the sleeve.